RB26powered74zcar Posted January 24, 2007 Share Posted January 24, 2007 I need some help with my alt hook up, as I'm not so smart when it comes to wiring.... I finally got my car up and started, but have a issue with the charging. I'm using a after market ecu with a laptop to show me the vitals while I have it idle in the driveway for now. My laptop shows the alt is putting out 13.5 - 14v but soon as my electric fan kicks on (thermostat switch turns it on @ 190deg) the charging system goes bananas... it jumps to 17 - 18v which throws my standalone ignition into a frenzy and makes it run like crap. What would cause the alt to spike up the charging volts like that? I must admit, I hooked the RB alt up to the Z wire harness very simple. I just hooked up the charge wire in the Z harness (the wire that was hooked to the Z alt + post) to the + post on the RB alt, then ran that to the + on the starter, then to the + post on the battery. I did not use any of the old external voltage regulator wires, and did not use the T connector (sensing and lamp I think). Basically, I think I have the RB alt hooked up like this: the heavy gauge wire in the Z car harness that goes to the power post on the Z alternator, is hooked to the power post on the RB alt, then I ran a heavy wire from there, off the alt to the starter, then from there to the positive on the battery. It works cause my laptop shows it charging at 13.5+ volts while running in the driveway. The problem I have is the spike when the fan comes on. What do ya think guys? Did I goof up the hook up of the RB alt? if so, how do I fix it? thanks in advance - joel Quote Link to comment Share on other sites More sharing options...
stony Posted January 24, 2007 Share Posted January 24, 2007 Hmm that sounds wierd. If you dont have the alt wired into the z voltage reg maybe its just the alternator is bad???? I had a similar problem with mine... may have been a indication problem BUT. when i started mine up the Voltage gage in the car would only read 11-12 volts but when i revved the engine it would pop up to 13-14 volts and stay there. It was almost like the 2 regulators were fighting each other. and when i revved the engine the alternator internal reg took over. Matt wired mine when i was in japan and im pretty sure it was wired into the stock harness the same way the stock alt was. Which I think means i had 2 regulators in the system. Post your question in the yahoo group.. im sure you will get an answer quick. Im pretty sure Matt wrote something up in hybridz somewhere on who to wire everything up if you want to check your setup. Quote Link to comment Share on other sites More sharing options...
J Taylor Posted January 24, 2007 Share Posted January 24, 2007 Stony: Mine does the same thing. It doesn't start charging until I rev it up to about 3000 or so rpm. If you ever figure out why that is let me know. Joel: I had an issue at first as well. When I got it together with the rb25 alternator it would overcharge....like 14.7-15.0. My neighbor was working on a rb26 swap into his z32 at the time so I borrowed his to try and it fixed the problem.....charged around 14.0. I took the old alternator to a repair shop who said it was fine.....apparently there are different alternators on the rb's....the one I have had to be wired slightly different or something. It works fine on my friends z32 though. When I bought my next motor I tried that alternator and same thing.....overcharging. I know this probably doesn't help you but maybe something to think about. JT Quote Link to comment Share on other sites More sharing options...
stony Posted January 24, 2007 Share Posted January 24, 2007 What i think is going on is the internal reg in the alternator is not putting out enough at idle to kick over the stock Z voltage regulator until you rev it. then it kicks over and stays over allowing the alternator to change the system. Quote Link to comment Share on other sites More sharing options...
Z U L8R Posted January 24, 2007 Share Posted January 24, 2007 my 280z has a thick white/blue and a small white/red wire off the Z harness, i cut the plug off and wired it to my rb25's alternator which had a thick white/red and a thin blue wire on the rb's alt plug. i first wired up thick to thick and thin to thin and it wasn't charging correctly. so i put the thick white/red (Z) to the thin white/red (rb plug) and connected the other and it charged perfectly 14.2 v. i wired in my fan directly to the battery with an inline 30 amp fuse, and the thermostatic fan relay in between the ground, i grounded the relay and when the relay gets to the temp it needs it completes the circuit, when my fan cuts on it drops .2 volts but other than that no problems Quote Link to comment Share on other sites More sharing options...
Pop N Wood Posted January 24, 2007 Share Posted January 24, 2007 I have no direct experience with RB alternators. However, if it is an internally regulated alternator there should be a voltage sensing wire coming out of the alternator. This wire is usually hooked into the wiring harness near the volt gage somewhere. The voltage regulator in the alternator will then adjust the alternator output to maintain a constant 14 or so volts at that point. If you have the voltage sensing wire hooked to the wrong place, say somewhere near the fan electrical inlet, then the large current draw through the fan wire will cause a voltage drop along the wire. The voltage regulator will then pump up the alt output to maintain 14 volts at the end of the wire. This means the alternator may have to put out 17 volts to compensate for the say 3 volt drop along the wire. So my recommendation is to check where the voltage sensing wire from the alternator is going and see if the alternator is maintaining the correct voltage at that point. Quote Link to comment Share on other sites More sharing options...
RB26powered74zcar Posted January 24, 2007 Author Share Posted January 24, 2007 I have no direct experience with RB alternators. However, if it is an internally regulated alternator there should be a voltage sensing wire coming out of the alternator. This wire is usually hooked into the wiring harness near the volt gage somewhere. The voltage regulator in the alternator will then adjust the alternator output to maintain a constant 14 or so volts at that point. If you have the voltage sensing wire hooked to the wrong place, say somewhere near the fan electrical inlet, then the large current draw through the fan wire will cause a voltage drop along the wire. The voltage regulator will then pump up the alt output to maintain 14 volts at the end of the wire. This means the alternator may have to put out 17 volts to compensate for the say 3 volt drop along the wire. So my recommendation is to check where the voltage sensing wire from the alternator is going and see if the alternator is maintaining the correct voltage at that point. Thank You my friend, that is the most likely culprit.... I will check to see where I goofed up on that two prong "T" plug. Thanks to all who have helped so far.... I will keep you updated. I hate that feeling of having to wait to solve a silly issue before you can take your brand new project out for a nice test run. I can't wait to put these twins on full song... Quote Link to comment Share on other sites More sharing options...
ZDrifter Posted January 25, 2007 Share Posted January 25, 2007 Mine does the same exact thing too, 10-12 volts on start up, rev it up and it jumps to 14 and holds till you turn the car off. Im really interested now Quote Link to comment Share on other sites More sharing options...
Evan Purple240zt Posted January 25, 2007 Share Posted January 25, 2007 My 240sx does not show that behavior. Just thought I would add this since I am using all the stock harnesses for the 240sx on the alternator. Sounds like a good suggestion on the sensing wire, I had an alternator in my S30 (gm alt) and had the sensing wire in the wrong place and got wierd charging issues. Evan Quote Link to comment Share on other sites More sharing options...
stony Posted January 25, 2007 Share Posted January 25, 2007 240SX didnt have an external voltage regulator. The older Zs did. i relly thing thats what casues that. Quote Link to comment Share on other sites More sharing options...
RB26powered74zcar Posted January 25, 2007 Author Share Posted January 25, 2007 I am getting so aggravated.... I still can't figure out how to get this dam alt hooked up to where it holds a semi steady charge voltage, with out jumping up to 16, 17, 18v when I rev it. I've ended up unhooking everything on the alt but a large wire that I have going from the batt post on the alt, to the positive terminal on the batt, then to the starter. It charges this way, but is not allowing the internal reg to control the voltage when I rev it. I need some one to help me bypass the Z car wire harness, and tell me how to hook up a alt (hot wire style) directly to a battery where it will charge the battery, and have the internal reg work correctly. I'm pretty sure its the brown plug on the RB alt thats causing my trouble, as I believe thats the two wires in the brown plug that the alt uses to sense the voltage, and there for regulate it..... Can some body PLEASE help me figure this out. I need basically to wire this alt from scratch, w/o using any wires off the old externally regulated 74 harness..... Please, no guessing here. If you know for sure how to hook up a internal reg alt from scratch, PLEASE CHIME IN... thanks - joel Quote Link to comment Share on other sites More sharing options...
Z U L8R Posted January 25, 2007 Share Posted January 25, 2007 just a question, if the alternator is fine until the electric fan comes on, if you remove the fan from the equation all together is everything else perfect? if so, instead of jumping a wire from the alternator charging stud to the Z's fuseblock then jumping it to your battery then battery to starter etc etc etc. what if you wire the fan in differently? if the problem exists whether the fan is in the car or not then this is a moot point, and your alternator just may have a bad internal regulator, could you borrow someone elses alternator? Quote Link to comment Share on other sites More sharing options...
RB26powered74zcar Posted January 25, 2007 Author Share Posted January 25, 2007 just a question, if the alternator is fine until the electric fan comes on, if you remove the fan from the equation all together is everything else perfect? if so, instead of jumping a wire from the alternator charging stud to the Z's fuseblock then jumping it to your battery then battery to starter etc etc etc. what if you wire the fan in differently? if the problem exists whether the fan is in the car or not then this is a moot point, and your alternator just may have a bad internal regulator, could you borrow someone elses alternator? I thought of that so I unhooked the fan wiring and it does the same thing..... I can go out and start it up with a cold motor and it acts the same. I don't have another alt at the moment. Its got to be the way I don't have the sensing wire (brown plug) hooked up right.. Quote Link to comment Share on other sites More sharing options...
motoman Posted June 8, 2011 Share Posted June 8, 2011 (edited) So i was reading this post i know it is from a long time ago, but it got me thinking. So we know that the Alternator on the Rb is a three wire internal regulator. Which means there is three places for wires to connect at; one for the charge stud, ignition sense, and battery sense. Four if you include a ground. The ground most likely isn't going to cause a problem unless its not a clean connection. My guess on what is giving you the problem is the sensing lead. This needs to run from your power distribution center or the + battery lead back to the correct position on the alt. I'm not sure which lead it is on the alt but is one in the brown plug. Edited June 8, 2011 by motoman Quote Link to comment Share on other sites More sharing options...
wrcbonk Posted June 13, 2011 Share Posted June 13, 2011 Following up on what Motoman said, I copied this from Bishopsrock's wiring thread: On my alternator the turquoise with a red band was the 'S' Sensing connection and so I cut this at the plug and extended it to the battery +terminal. Bonk Quote Link to comment Share on other sites More sharing options...
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